1. Field of the Invention
The present invention relates generally to an optical device, and more particularly to an optical storage device using the optical device.
2. Description of the Related Art
An optical disk is used as a medium for recording/reproducing voice, character, image information, etc. in the form of compact disk (CD), CD-ROM, CD-R, digital versatile disk (DVD), magneto-optical disk (MO disk), mini-disk (MD), etc. Such an optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data from/to the optical disk by means of an optical pickup.
The optical pickup in a recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly including a laser diode, a polarizing beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, and a movable optical assembly including a carriage and an actuator mounted on the carriage and having an objective lens. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor (VCM).
A write-power laser beam emitted from the laser diode of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the polarizing beam splitter, next reflected by a beam raising mirror of the actuator, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. Reflected light from the optical disk is first collimated by the objective lens, next reflected by the polarizing beam splitter, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.
To further reduce the size of the optical pickup, there has been developed such a configuration that diverging light emitted from a laser diode is directly incident on a polarizing beam splitter. In this optical pickup, converging light reflected from an optical disk is returned to the polarizing beam splitter, and is partially reflected by the polarizing beam splitter. Accordingly, a lens for focusing the light reflected by the polarizing beam splitter onto a photodetector can be omitted. To make the optical pickup more compact, there has extensively been developed an integrated optical pickup (optical head) configured by integrating a laser diode, polarizing beam splitter, optical output monitor, servo detection system, RF signal detection system, and preamplifier circuit.
In these optical pickups, the angle of incidence of the diverging light on a polarization separating film of the polarizing beam splitter has a distribution, so that the transmittance and reflectance of the polarization separating film, the phase difference between polarized light components, and the polarization direction have respective distributions. To prevent a degradation in characteristics due to these distributions, the aperture diameter of the diverging light emitted from a light source is reduced to suppress the influence of the distribution of the incidence angle to the polarizing beam splitter. Accordingly, in a conventional read-only mini-disk drive or the like wherein no problem arises even when a utilization efficiency of light is low, an optical pickup having a configuration that the diverging light is incident on the polarizing beam splitter is adopted to realize the size reduction.
In a rewritable optical disk drive, high-speed recording and high-speed transfer are required and it is therefore necessary to increase a utilizable quantity of light from a light source. In reducing the size of such an optical disk drive, the distribution of optical characteristics due to the distribution of the incidence angle to the polarization separating film of the polarizing beam splitter may suppress the performance of the disk drive. Referring to FIG. 1, there is shown a distributed condition of a traveling direction of diverging light 4 emitted from a laser diode 2. The laser diode 2 emits P-polarized diverging light with respect to a polarization separating film 8 of a polarizing beam splitter (PBS) 6. In this description, P-polarized light is defined as linearly polarized light such that the vibrational direction of an electric vector is in the incidence plane of the diverging light 4 to the polarization separating surface 8, and S-polarized light is defined as linearly polarized light such that the vibrational direction of an electric vector is perpendicular to the incidence plane of the diverging light 4 to the polarization separating film 8.
As shown in FIG. 1, the P-polarized diverging light 4 emitted from the laser diode 2 spreads both in an X direction and in a Y direction. A light ray (principal ray) 4a on the optical axis of the diverging light 4 is incident on the polarization separating surface (BS surface) 8 at an incidence angle of 45°. As shown in FIG. 3, the PBS 6 is arranged so that the polarization direction of the diverging light 4 (P-polarized light) emitted from the laser diode 2 is parallel to an ideal incidence plane 10 to the BS surface 8 of the PBS 6. The ideal incidence plane 10 is a plane defined below. That is, the ideal incidence plane 10 means a plane containing both the light ray 4a and reflected light obtained by reflection of the light ray 4a on the BS surface 8 of the PBS 6. Therefore, the ideal incidence plane 10 is perpendicular to the BS surface 8.
Since the diverging light 4 emitted from the laser diode 2 spreads both in the X direction and in the Y direction as mentioned above, a peripheral light ray 4b of the diverging light 4 intersecting the X direction is not parallel to the ideal incidence plane 10, but forms a certain angle θ. Accordingly, the incidence plane of the light ray 4b to the BS surface 8 forms the angle θ with respect to the ideal incidence plane 10. The polarization direction of the diverging light 4 emitted from the laser diode 2 is fixed in such a manner that the polarization direction of the light ray 4a on the optical axis of the diverging light 4 is the same as the polarization direction of the peripheral light ray 4b of the diverging light 4. Accordingly, the polarization direction of the light ray 4b and its incidence plane form the angle θ. As defined above, the P-polarized light is linearly polarized light whose vibrational direction of an electric vector is in the incidence plane, and the polarization direction of the light ray 4b is not parallel to the incidence plane. Therefore, the light ray 4b includes a slight amount of S-polarized light component to the BS surface 8 in addition to the P-polarized light.
As shown in FIG. 4, the transmission axis (P-polarization direction) of the BS surface 8 and the polarization direction of the incident light 4b form the angle θ, so that the polarization direction of the light ray 4b transmitted through the BS surface 8 undergoes rotation by the angle θ. In a cross section 12 of the diverging light 4 shown in FIG. 2, the polarization direction 14 in a central region 12a is a P-polarization direction, and the polarization direction 14′ in a side region 12b spreading in the X direction is rotated according to the spreading of the diverging light. Thus, the polarization direction is distributed in the cross section 12 of the diverging light 4 as shown in FIG. 2. That is, the rotational angle of the polarization direction has an angular distribution in a direction perpendicular to the ideal incidence plane 10. This distribution of the polarization direction similarly arises also in a beam spot focused on a medium. In FIG. 4, reference numeral 15 denotes a traveling direction of light.
Referring to FIG. 5, there is shown a polarization direction in an optical path of a laser beam transmitted through a polarizing beam splitter (PBS) and returned thereto after reflection on a medium 20. That is, the laser beam transmitted through the polarizing beam splitter is converted into collimated light 16 by a collimator lens, and next focused on the medium 20 by an objective lens 18. The laser beam focused on the medium 20 is reflected on the medium 20, and reflected light from the medium 20 is returned through the objective lens 18 and the collimator lens to the polarizing beam splitter. The polarization direction in the forward path after transmission through the PBS to reflection on the medium 20 is the same as the direction of an incidence plane to the PBS. The polarization direction in the backward path after reflection on the medium 20 to incidence on the PBS is geometrically the same as the polarization direction in the forward path. However, the peripheral light rays of the collimated light 16 in the backward path are converged by the collimator lens to enter the PBS, so that the polarization direction of the peripheral light rays of the converging light from the collimator lens forms an angle with respect to the direction of an incidence plane to the PBS in the backward path. As a result, the polarization plane of the collimated light 16 in the backward path is rotated in appearance. In the case that light quantity distribution in the backward path is symmetrical, this rotation of the polarization plane causes no problem. In actual, however, light quantity distribution in the backward path is nonsymmetrical, causing a degradation in optical characteristics.
As shown in FIG. 6, a guide groove 22 for tracking is formed on a rewritable optical disk. Laser light directed on a track 24 formed on the disk is diffracted by the guide groove 22 adjacent to the track 24 to generate diffracted light 26 as shown in FIG. 6. This diffracted light 26 interferes with reflected light 28 from the track 24 to form a bowl-shaped bright/dark pattern. Accordingly, when a beam spot on the disk moves across the guide groove 22, the bowl-shaped bright/dark pattern is varied to cause apparent rotation of a polarization plane. As a result, there arises a problem that a tracking error signal may leak into a reproduction signal based on the rotation of a polarization plane in reproducing information recorded on a magneto-optical recording medium.
FIG. 7 shows the leakage of a tracking error signal (TES) into an MO2T signal as a shortest mark of a magneto-optical signal (MO signal). In the case that a beam spot follows a track as in detection of magneto-optical information, no problem arises. However, if there is dust on the medium or defect of the track, noise due to such dust or defect may leak into the MO signal, so that the MO signal cannot be detected. Further, an optical disk formed with a wobble track 30 as shown in FIG. 8A or a one-sided wobble track 32 as shown in FIG. 8B for the purpose of address detection is known as a rewritable optical disk. The wobble track is a wavy track whose center is transversely deviated by a minute distance from the center of a normal track at a predetermined frequency, e.g., 22 kHz, and an address on the disk can be determined according to the phase of the wavy track. Accordingly, it is unnecessary to record addresses on the disk, and more information can therefore be recorded. However, since the wobble track 30 or the one-sided wobble track 32 has a high frequency band, tracking servo does not follow the meandering by the wobbling, so that the detection of the MO signal is difficult because of the leakage of noise into the MO signal.
A method of suppressing a disturbance due to the leakage of noise into the MO signal is disclosed in Japanese Patent Laid-open No. Hei 7-57320. In this method, a laser diode, PBS, and optical disk are arranged so that the track direction on the optical disk and the direction of an incidence plane of diverging light from the laser diode to the PBS are perpendicular to each other. However, the direction of divergence angle distribution of the light from the laser diode, the polarization direction of the light, and the spectral characteristics of the PBS are fixed. Another method of suppressing a leak signal into the MO signal by masking a part of a return optical path in an MO detection system is proposed in Japanese Patent Application No. Hei 12-40849. In this method, it is difficult to adjust a mask position.